There is conventionally provided a data communication system for vehicles, the system including (i) a relay apparatus receiving a rewriting target program from a rewriting apparatus and (ii) an electronic control unit (hereinafter, referred to as ECU) receiving the rewriting target program from the relay apparatus and executing a rewriting operation with the rewriting target program. Such a data communication system for vehicles needs to detect whether a rewriting operation with a rewriting target program is executed correctly or incorrectly from a risk that the rewriting operation using the rewriting target program may be executed incorrectly.
A method detecting whether a rewriting operation with a rewriting target program is executed correctly or incorrectly is disclosed in Patent literature 1, for instance. In the method disclosed in Patent literature 1, a plurality of ECUs each generate a verification message by using an identical algorithm, transmitting it to the server. When the values contained in respective verification messages received from the plurality of ECUs are different, or when a failure is determined in receiving a verification message which should be received, the server identifies an incorrect rewriting operation with the rewriting target program.